Dual Connectivity (DC) refers to the operation in which a given user equipment (UE) consumes radio resources provided by at least two different network points (such as, for example, a Master eNodeB (MeNB) and Secondary eNodeB (SeNB)) connected with non-ideal backhaul while in RRC_CONNECTED. A UE in DC maintains simultaneous connections to the MeNB (often, the anchor node) and the SeNB (often, the booster node). The MeNB controls the connection and handover of SeNB. No SeNB standalone handover is defined for Rel-12. Signaling in MeNB is needed when the SeNB changes. Both the MeNB and SeNB can terminate the control plane connection towards the UE, and can thus be the controlling nodes of the UE. The UE reads system information from the MeNB. In addition to the MeNB, the UE may be connected to one or several SeNBs for added user plane support. The MeNB and SeNB are connected via the Xn interface, which is currently selected to be the same as the X2 interface between two eNBs.
FIG. 1A is a schematic diagram of a system under a dual connectivity operation. More particularly, FIG. 1A illustrates one or more UEs 110A, 110B, and 110C, and one or more network nodes 115A, 115B, and 115C. Network node 115A may be an MeNB, and network node 115B may be an SeNB. UE 110A may be connected to only MeNB 115A, while UE 110B may be connected to both MeNB 115A and SeNB 115B. UE 110C may be connected to network node 115C. Although FIG. 1A illustrates only one SeNB 115B connected to UE 110B, in certain embodiments more than one SeNB can serve UE 110B. Typically, a UE, such as UE 110B, may be configured with at least a primary component carrier (PCC) from MeNB 115A and a PCC from SeNB 115B. The primary serving cells on PCCs from MeNB 115A and SeNB 115B are generally called a primary cell (PCell) and a primary secondary cell (PSCell), respectively. UE 110B may also be configured with one or more secondary component carriers (SCCs) from MeNB 115A and SeNB 115B. The serving cell on SCC is called a secondary serving cell (SCell). Often, the PCell and PSCell operate or serve the UE independently.
FIG. 1B is a schematic diagram of a system under a dual connectivity operation. More particularly, FIG. 1B illustrates a pair of network nodes 115A and 115B. Network node 115A may be an MeNB, and network node 115B may be an SeNB. FIG. 1B also illustrates two UEs 110D and 110E. UE 110D may be a DC capable UE, and UE 110E may be a legacy UE. DC is a UE-specific feature. Thus, a network node can support a dual connected UE and a legacy UE at the same time. For example, SeNB 115B may support UE 110D using dual connectivity operation and legacy UE 110E at the same time.
As described above, the roles of MeNB 115A and SeNB 115B are defined from a UE perspective, which means that a network node that acts as an MeNB to one UE may act as an SeNB to another UE. Similarly, though the UE reads the system information from the anchor node (i.e., MeNB 115A), a node acting as a booster (i.e., SeNB 115B) to one UE, may or may not distribute system information to another UE. The MeNB and SeNB may perform a variety of functions. For example, MeNB 115A may provide system information, terminate the control plane, and can terminate the user plane. SeNB 115B can terminate the control plane or terminate only the user plane.
In one application, DC allows a UE to be connected to two network nodes to receive data from both network nodes to increase its data rate. This user plane aggregation achieves similar benefits as carrier aggregation using network nodes that are not connected by a low-latency backhaul/network connection. Due to this lack of low-latency backhaul, the scheduling and HARQ-ACK feedback from the UE to each of the network nodes will need to be performed separately. That is, it is expected that the UE shall have two UL transmitters to transmit UL control and data to the connected network nodes. In light of this, DC becomes a special case of carrier aggregation. Carrier aggregation is one of the ways of increasing the per user throughput for users with good channel conditions and with the capability of receiving and transmitting at higher data rate. A user can be configured to two or three (or more) simultaneous bands in downlink (DL) and/or in uplink (UL).
FIG. 2A is a schematic diagram of a network node 115 that is capable of running four different cells at the same time. Each arrow 202-216 represents a cell in either the UL or DL. For example, arrows 202, 206, 210, and 214 are in the DL, and arrows 204, 208, 212, and 216 are in the UL. These cells may be operated in different bands, or they may be operated in the same band. In Release 8, for example, and as shown in FIG. 2A, only one cell is used for communication between network node 115 and UE 110, as shown by adjacent shaded arrow 210 in the DL and arrow 212 in the UL.
In carrier aggregation terms, the cell where UL is allocated for a UE 110 is the primary cell (PCell), while the other aggregated cell is the secondary cell (SCell). PCell and SCell combinations are UE specific. Other carrier aggregation cases based on the number of CCs in DL and/or UL may exist. Examples of these cases are described in more detail below.
FIG. 2B is a schematic diagram of a network node 115 running two cells activated for one user equipment 110. More particularly, FIG. 2B illustrates a version of DL carrier aggregation with two DL CCs 202, 210, and one UL CC 204. As shown in FIG. 2B, UE 110 is configured to receive in 2 DL bands simultaneously, while still using UL in only one of the bands. The UL allocation in this case is arbitrary, meaning that either of the bands (e.g., 204 or 212) can be used for UL transmission.
FIG. 2C is a schematic diagram showing a network node 115 running three downlink bands allocated to a user equipment 110. More particularly, FIG. 2C illustrates a version of DL carrier aggregation with three downlink CCs 202, 206, 210, and one UL CC 204. Similar to the two DL CC configuration described above in relation to FIG. 2B, the UL can be allocated to any of the bands (e.g., 204, 208, 212). In some cases, a second UL CC could be allocated to UE 110.
FIG. 2D is a schematic diagram showing a network node 115 running uplink carrier aggregation. More particularly, FIG. 2D illustrates a version of UL carrier aggregation with two UL CCs 204, 208, and two DL CCs 202, 206. In the case of UL carrier aggregation, PCell and SCell definitions are still UE specific.
Depending on the carrier frequency, or depending on the physical eNB deployment, the deployment of a carrier aggregation-enabled system can be very different. Some example carrier aggregation deployment scenarios are described in more detail below in relation to FIGS. 3A and 3B.
FIG. 3A is a schematic diagram showing an example carrier aggregation deployment scenario. More particularly, FIG. 3A shows that frequency 1 (F1) and frequency 2 (F2) cells are co-located and overlaid, but F2 has smaller coverage due to larger path loss. Only F1 provides sufficient coverage, and F2 is used to improve throughput. Mobility is performed based on F1 coverage. As an example scenario, F1 and F2 are of different bands, e.g., F1={800 MHz, 2 GHz} and F2={3.5 GHz}, etc. It is expected that aggregation is possible between overlaid F1 and F2 cells.
FIG. 3B is a schematic diagram showing another example carrier aggregation deployment scenario. In FIG. 3B, F1 provides macro coverage, and on F2 Remote Radio Heads (RRHs) are used to improve throughput at hot spots. Mobility is performed based on F1 coverage. In such a scenario, it is likely that F1 and F2 are of different bands, e.g., F1={800 MHz, 2 GHz} and F2={3.5 GHz}, etc. It is expected that F2 RRHs cells can be aggregated with the underlying F1 macro cells.
One consideration in dual connectivity operation is the capability of a particular UE. UEs may be capable of supporting certain number of CCs, based on the UE RF architecture. For example, the maximum number of CCs that a UE can support may be 5. During connection setup, the UE reports its capability to the network. The MeNB and SeNB(s) configure the UE independently regarding the number CCs from MeNB and SeNB(s). A UE can handle a maximum of 5 CCs while connected to arbitrary number of nodes. The limit on the number of CCS is set by UE capability. That is, the UE supports 2, 3, 4, or 5 CCs. When MeNB and SeNB configure the UE independently, there is a possibility that the total number of CC requests may become higher than the limit. The individual node (MeNB and SeNB) may not know the current total number of CCs that the UE is configured with. When MeNB and SeNB have little coordination between them, then there is a possibility that the nodes may configure the UE to handle CCs that the UE cannot handle. Thus, there is a need for an improved method of configuring CCs for UEs in DC operation.